Mobile telecommunications can be defined as mobility-added telecommunications. Mobility includes terminal mobility that removes the limits on the length of a phone line, user mobility that enables a user to communicate using a terminal while moving freely, and service mobility that provides data and video services as well as conventional voice service by utilizing wide-band wireless channels.
With the development of mobile communications technology, mobile terminals such as cellular phones, PCS phones, and PDAs that implement mobile telecommunications are becoming smaller and lighter.
Telecommunications networks provide various services for mobile subscribers. In case a subscriber cannot answer an incoming call and the caller leaves a voice message, the called subscriber can get the voice message afterwards through the voice mail service. A subscriber can use his/her voice to dial a number after registering his/her voice in the mobile terminal having the function of the voice-activated dialing service. The short message service (SMS) enables a subscriber to receive various messages and information through the screen of his/her mobile terminal. Also, the short message service notifies a subscriber of the arrival of a voice mail for the subscriber as soon as a caller leaves a voice message when the called subscriber uses the voice mail service.
When a call is received, a called subscriber is able to know the caller's number immediately by the calling number display service that provides the number of every received call.
The conventional calling number display service provides the numbers of normally connected calls but cannot provide the numbers of calls that were tried but not received by the called subscriber because the called subscriber's terminal is power-off, the called subscriber's line is busy, or the called subscriber is out of the service coverage.